Fuel injectors for internal combustion engines commonly used solenoid operated valves to meter fuel under pressure either upstream of a manifold type distribution system or on an individual cylinder basis at a point near the intake valve. The former arrangement is commonly called "throttle body injection" and the latter is commonly called "multipoint injection".
More recently it has been discovered that the fuel metering function and an atomizing function can be achieved using an acoustically resonant structure which is periodically excited with an alternating current excitation signal. Although such structure may take various forms, it may be generally described as comprising the combination of a mechanical device, such as a catenoidal hornshaped injector body, and an electrical device such as a piezoelectric crystal or an arrangement of several such crystals. One combination pertinent to the invention described herein comprises a catenoidal horn having a ball check valve in the fuel flow path near the small tip of the horn and a pair of electrically parallel-connected piezoelectric crystals mechanically abutting the large end of the horn. When the crystals are excited by an alternating current pulse of controlled frequency and amplitude, the horn is set into resonant vibration to unseat the ball and permit a metered quantity of fuel to flow to the combustion chamber or chambers.
The successful use of an acoustic fuel injector requires the ability to precisely control the injected fuel quantity under varying operating conditions. Such control is, in great measure, affected by the degree to which the frequency of the excitation signal matches the mechanically resonant frequency of the acoustic structure; i.e., even a small mis-match results in decreased vibration amplitude at the tip of the horn where metering and atomization takes place. This is a difficult match to maintain because, as previously described, the resonant structure includes both electrical and mechanical components. Moreover, the resonant frequency of the structure is not constant; rather, it is known to vary significantly with temperature, load and contamination level. Unless the frequency of the excitation signal can be made to follow such variations in mechanical resonant frequency, precise fuel metering is not possible.
It is known, therefore, that the oscillator and the resonant structure may be electrically integrated such that the resonant structure forms part of the tuning circuit of the oscillator. The result is a form of self-tuning wherein changes in the mechanically resonant frequency of the structure due to temperature, load and contamination are automatically reflected into the oscillator excitation frequency. The deficiency of such systems lies in the failure to compensate the self-tuning function for static components which do not follow or change in proportion to the changes in mechanical resonance.